1. Field of the Invention
This invention relates generally to Gallium Nitride (GaN) semiconductors and their fabrication. Because of its direct band gap of 3.4 ev, GaN should be efficient for luminescence ranging from the visible blue to the ultra-violet (uv) part of the electromagnetic spectrum. The present invention relates to the fabrication and utility of p-Type Gallium Nitride (pGaN) and use of pGaN in semiconductor devices such as uv and blue light-emitting diodes and lasers.
2. Description of Related Art
Many researchers have attempted to make p-doped GaN without success. Two basic methodologies have formed the focus of past research. One method is chemical vapor deposition (CVD) combined with in-situ Magnesium (Mg) doping. This method has been investigated by Amano, et al. (H. Amano, M. Kito, K. Hiramatsu, and I. Akasaki, Jpn. J. Appl. Phys. 28:L2112, 1989), Nakamura et al. (S. Nakamura, T. Mukai, and M. Senoh, Jpn. J. Appl. Phys. 30:L1998, 1991), and Goldenberg (B. Goldenberg, J. D. Zook, R. J. Ulmer, Appl. Phys. Lett. 62:381, 1993). These researchers obtain p-doped GaN with hole mobilities, .mu., of about 10 cm.sup.2 /V-sec. These hole mobilities reflect significant concentrations of deep level impurities that will limit utility of the material in optoelectronic devices. There are other limitations associated with CVD techniques in making GaN. The source of nitrogen ion for CVD growth is typically ammonia gas (NH.sub.3). The presence of H decreases the effective hole carrier concentration in GaN S. Nakamura et al., "Hole compensation mechanism of p-type GaN films", Jpn J Appl. Phys, 31: 1258-1266 (1992). It is thought that hydrogen, which can act as an electron donor, binds to Mg or other acceptors forming a complex that no longer acts as an acceptor. Furthermore, CVD subjects the growing overlayer and the substrate to high temperatures, in the range of 1200.degree. C. This restricts the choice of substrates to one that can withstand the high temperatures. The high temperatures are undesirable for layered structures because interdiffusion between layers increases with temperature.
Moustakas has reported an unpublished presentation that he obtained pGaN using molecular beam epitaxy (MBE) with an electron-cyclotron-resonance (ECR) nitrogen plasma source and in-situ Mg doping. He was only able to obtain low hole mobilities of about 1 cm.sup.2 /V-sec. One problem with an ECR nitrogen source is that the Nitrogen has a wide range of energies when it impinges on the substrate. ECR, which uses microwaves to produce N.sub.2.sup.+, N.sup.+, and their excited states, N.sub.2 *, N* also produces chamber contaminants because the nitrogen plasma ions can strike the interior walls of the chamber thus releasing secondary materials from the chamber walls. One of the main obstacles to fabricating high quality GaN crystals for electronic applications has been control over the concentration of background contaminants and dopants.
Recently Powell (R. C. Powell et al., J. Appl. Phys. 73(1):189, 1993) produced undoped material with a reduced electron and contaminant concentration using plasma/ion-assisted molecular beam epitaxy. He achieved a free electron concentration of 8.times.10.sup.13 /cm.sup.3. However he has not reported successful synthesis of pGaN.
GaN has a direct band gap of 3.4 eV which should be efficient for luminescence ranging from the visible blue to ultra-violet (uv) part of the electromagnetic spectrum. Because it is well known how to fabricate n-type GaN, it would be very desirable to have p-type GaN. Recombination of electrons and holes (either band-to-band or defect mediated) at the p-n junction could then be used as a source of luminescence. It would be even more desirable to have pGaN with high hole mobilities. It would be still more desirable to be able to synthesize pGaN predictably. It wold be even more desirable to predict concentrations of holes and levels of hole mobilities when synthesizing pGaN. It would be additionally advantageous to be able to synthesize GaN at lower temperatures than those required by conventional deposition techniques, such as CVD, so that practical substrates, that are harmed at high temperatures, could be employed. An even further benefit of growing GaN at the lower temperatures is reduced interdiffusion between layers in a layered structure.